Magnetism and superconductivity in R_1.5Ce_0.5MSr₂Cu₂O _10-δ (M = Ru and Fe) compounds

It is shown here, that the superconducting (SC) R_1.5Ce_0.5RuSr₂Cu₂O _10-δ (RCeRuSCO, R = Eu and Gd) materials (T_c ∼ 32 and 42 K) are also antiferromagnetically (AFM) ordered at T_N(RU) ∼ 122 and 180 K, respectively, thus, T_N ≫ T_c, a trend which is contrary to that obtained in "magnetic-SC" intermetallic systems. Mössbauer spectroscopy (MS) on 0.5% ⁵⁷Fe doped samples shows that all Fe ions reside in the Ru site which is magnetically ordered, whereas SC is confined to the CuO₂ planes. On the other hand, for Y_1.5Ce_0.5FeSr₂Cu₂O₉ (YCeFSCO) no SC is found and the Cu-O planes order AFM at T_N(Cu) = 31 K. MS studies reveal two in equivalent Fe sites, and that Fe resides predominantly (60%) in the Cu(1) site. In both sites, the Fe does not order magnetically, and the low T_N(CU) obtained is due to frustration of the Cu moments by the presence of Fe. T_N is sensitive to oxygen concentration and shifts toward 260 K when oxygen is depleted.